Semiconductor devices such as Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are commonly used as power devices in applications, such as automotive electronics, power supplies, telecommunications, which applications require devices to operate at currents in the range of tenths up to hundreds of amperes (A).
Conventionally, by applying a voltage to the gate electrode of a MOSFET device, the device is turned on and a channel will be formed connecting the source and the drain regions allowing a current to flow. A lightly doped drift region is formed between the drain region and the channel. The drift region is required to be lightly doped in order to lower the maximum electric field that develops across the PN junction (p-body/n-epi) and thus, to ensure a high breakdown voltage. Once the MOSFET device is turned on, the relation between the current and the voltage is nearly linear which means that the device behaves like a resistance. The resistance is referred to as the on-state resistance Rdson.
Typically, MOSFET devices with low on-state resistance Rdson are preferred as they have higher current capability. It is well known that the on-state resistance Rdson may be decreased by increasing the packing density of a MOSFET device i.e. the number of base cells per cm2. For example, a hexagonal MOSFET (HEXFET) device comprises a plurality of cells, each cell having a hexagonal polysilicon gate and source region forming vertices of the hexagonal polysilicon gate, and has a high packing density e.g. 105 hexagonal cells per cm2. Usually, the smaller the size of the cells, the higher is the packing density and thus, the smaller the on-state resistance. Therefore, many improvements to MOSFET devices are aimed at reducing the size of the cells.
However, it is well known that the breakdown voltage of MOSFET devices increases as the on-state resistance Rdson of the devices increases. Thus, there is a trade-off between reducing Rdson and having a high enough break down voltage BVdss.
In an attempt to reduce the on-state resistance Rdson of a MOSFET device whilst not impacting significantly the breakdown voltage of the device, it has been proposed to introduce multilayer structures in the epitaxial region of the device. These are known as super junction structures.
An article entitled ‘A Novel High-Voltage Sustaining Structure with Buried Oppositely Doped Regions’ by Xing Bi Chen, Xin Wang and Johnny K. O. Sin, in IEEE Transactions on Electron Devices, Vol. 47. No. 6, June 2000 describes a super junction structure having buried floating regions in the drift region of the MOSFET device connected together at the edge termination. In the case of p-type buried floating regions in a n-type drift region, due to the negative charges in the depleted p-type buried floating regions, a large part of the flux induced by the positive charges of the depleted n-drift region are terminated on the buried floating regions so that the electric field intensity is not allowed to accumulate throughout the entire thickness of the drift region. In other words, when the device is in an off state, the potential drop is distributed uniformly in the drift region due to the charge balance across the uniformly distributed buried floating regions and with the result that the peak electric field which develops decreases allowing the voltage capability of the device to be increased. This means that a larger doping concentration can be used in the drift region without producing a high peak field. Since a larger doping concentration in the drift region can be used, the on-state resistance Rdson is reduced. Thus, by using buried floating regions, the resistivity of the drift region can be made smaller than that of a conventional MOSFET device with the same breakdown voltage and therefore, the on-state resistance Rdson can be reduced.
The super junction structures are aimed at improving the voltage capability of the MOSFET device in the active area of the device. The active area is surrounded by a termination area which extends from the active area to the edge of the device (i.e. the edge of the die). A role of the termination area is to provide protection structures which protect the last PN junction in the active area, for example the junction between the last p-type body region and the n-type epitaxial layer of a MOSFET device, from the effects due to the junction curvature effect when the device is in an off state. Without some form of protection, due to the junction curvature effect, the distribution of the potential lines is curved around the last PN junction and a peak electric field develops at the junction near the surface of the epitaxial layer which, when the peak electric field exceeds a critical electric field for the device, is high enough to cause impact ionization avalanche-near the surface. This results in the breakdown voltage capability of the termination area being less than the active area. In order not to reduce the overall breakdown voltage capability of the MOSFET device, there is therefore a desire to increase the breakdown voltage of the termination region such that the breakdown voltage in the termination region is ideally substantially the same as the breakdown voltage in the active area.
Various termination structures have been developed, including field plate structures, field rings, guard rings and floating pylon regions such as those shown in U.S. Pat. No. 6,621,122.
Typically a field plate is formed by a conductive layer, for example a polysilicon layer in some MOSFET devices or a metal layer in diodes, extending several microns from the last PN junction over a field oxide layer in the termination area. In the MOSFET device shown in U.S. Pat. No. 6,621,122, the conductive layer is an extension of the source electrode. The conductive layer acts as a field plate and protects the last PN junction by spreading out the potential lines laterally away from the last PN junction and parallel to the field plate. The result is a reduction in the junction curvature effect at the last PN junction which results in an increase in the critical electric field at which breakdown occurs. However, when the MOSFET device is in an off state, junction curvature effect occurs at the edge of the field plate wherein the distribution of the potential lines is curved around the edge of the field plate and a peak electric field develops near the surface of the epitaxial layer at the edge of the field plate. The magnitude of the peak electric field at the surface of the epitaxial layer reduces the breakdown voltage capability of the termination area by an amount which depends on the thickness of the field oxide.
Thus, in order to avoid a reduction in the breakdown voltage, MOSFET devices having field plates extending into the termination area typically include additional termination structures, such as guard rings, in the termination area surrounding the active area of the device in order to reduce the junction curvature effect for example at the last PN junction and/or at the edge of the field plate.
The super junction MOSFET device described in U.S. Pat. No. 6,621,122 includes vertically connected p-type doped regions formed in pylons in the active area of the device and located under and in contact with p-type body regions. The p-type pylons are also formed in the termination area surrounding the active area and away from the conductive field plate edge towards the edge of the die. The p-type pylons are floating during device operation and increase the blocking capability of the MOSFET device. The width of the p-type pylons in the termination area is wider and their location is non-uniform compared to those p-type pylons in the active area of the device. Forming the p-type pylons from vertically connected p-type buried regions requires a complex manufacturing process which increases the cost of such devices.
U.S. Pat. No. 6,037,632 describes a super junction MOSFET device having p-type buried regions in the active area of the device and large width p-type guard rings used with p-type buried resurf guard rings having a lower doping concentration in the termination area of the device. Additional processing steps, such as forming additional mask layers, are required to create the lower doped buried resurf guard rings. This increases the cost of such devices.
U.S. Pat. No. 5,032,878 describes a termination structure for a high voltage planar device. The termination structure includes guard rings in the surface of the epitaxial layer with the guard rings furthest from the last PN junction being spaced further from each other compared to the guard rings closer to the last PN junction. In addition, an enhancement region of opposite conductivity type to that of the guard rings is formed between the guard rings to increase the punch-through voltage between the rings and thus, the breakdown voltage of the device.
Methods which use guard rings at the surface have a disadvantage in that the size of the device die increases by a significant amount.
There is therefore a need for an improved semiconductor device.